Issue #1069: Add Dict.drop, Dict.take and Dict.split

As per the issue description, the drop/2, take/2, and split/2 functions have been implemented in the Dict module.  

One potential addition (which is not implemented), is for the 3 functions to also accept single keys. So for example: 

```elixir
d = HashDict.new([a: 1, b: 2])
Dict.take(d, :a) # => [:a 1]
```

instead of having to always use a list of keys:

```elixir
Dict.take(d, [:a]) #=> [:a 1]
```

Not sure how important saving the effort of typing 2 characters (open and close bracket) is though ....